Conventional marine seismic surveying uses a seismic source and a number of streamers towed behind a seismic survey vessel. These streamers have sensors that detect seismic energy for imaging formations under the seafloor. Deploying the streamers and sources and towing them during the survey can be relatively straightforward when operating in open waters with moderate swells or the like.
However, marine locations covered by ice, debris, large swells, or other obstacles can make surveying more difficult, expensive, or even impossible. In icy waters, for example, the seismic survey vessel must break through ice and traverse waters filled with ice floes. The noise generated by ice impacts can also complicate the seismic record produced.
Additionally, the ice floes on the water's surface make towing the source and streamers more difficult and prone to damage. Any components of the system at the water's surface can encounter ice, become bogged down, and lost. In addition, any cables or towlines coming off the vessel even from slipways can collect ice at the surface, potentially damaging the cables or towline. Likewise, ice pulled under the hull and rising behind the vessel can shear away these cables and lines. Some approaches for performing seismic surveys in icy regions known in the art are disclosed in U.S. Pat. Nos. 5,113,376 and 5,157,636 to Bjerkoy. To date, however, the problems associated with marine seismic surveying in icy or obstructed waters have not been significantly addressed.
In addition to some of the physical challenges involved in surveying in arctic or icy regions, for example, variations in the earth's magnetic field in any give region of the earth can cause problems in land-based and marine-based seismic surveying. As is known, the earth's magnetic field can be described by seven parameters, including declination (D), inclination (I), horizontal intensity (H), the north (X) and east (Y) components of the horizontal intensity, vertical intensity (Z), and total intensity (F) measured in nanoTeslas. Most of the geomagnetic field (i.e., the main field) comes from the earth's outer core. Various mathematical models, such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM), can describe this main field and how it changes slowly over time. Although the changes of the field may be somewhat predictable, the geomagnetic field also varies due to currents inside the magnetosphere and the ionosphere and due to other variations that are less predictable.
The variations and changes in the geomagnetic field can affect seismic surveying in arctic regions as well as other locations. For example, compass readings from seismic survey equipment can be affected by differences in declination at arctic latitudes. As is known, magnetic declination represents an angle between magnetic north and true geographic north. The variation in declination depends on latitude and longitude and changes over time, and the variability in azimuth increases at arctic latitudes.
As will be appreciated, a compass reading can be corrected based on the magnetic declination (the angle between true north and the horizontal trace of the magnetic field) for the compass' location. To correct the compass' bearing, a true bearing is computed by adding the magnetic declination to the compass' magnetic bearing. Unfortunately, areas around the north and south magnetic poles can produce erratic or unusable compass readings, and some zones on the earth can have wide discrepancies in declination.
The published magnetic models of the earth invariably have errors or are not entirely accurate to a level of detail sometimes needed. Conventional marine-based seismic surveying can avoid problems by using a closed traverse in which GPS readings are consistently obtained by tail buoys on the streamers. In arctic exploration, however, the system cannot typically use a tail buoy due to ice floes so that the system cannot obtain GPS readings on a reliable basis. This makes tracking and locating seismic survey equipment and data more problematic for icy or obstructed waters.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.